I believe in order to book them more than about 3 months out it has to be part of an international connection. ANA and JAL both start selling pure domestic tickets approximately 3 months before departure. I'm not sure if the local LCCs follow the same practice or not. You can check JetStar, Vanilla, and Peach Aviation to see if they have anything that suits you, I am pretty certain they don't show up in Google flights.

Having said all of that, if you're not a resident of Japan then you might be eligible for very deeply discounted domestic tickets on the full-service carriers. JAL's requires you buy your ticket before departing for Japan and you must arrive on a round trip ticket of any OneWorld airline. ANA's allows you to fly to Japan on any airline at all. Both require you to show a foreign passport and tourist entry stamp. I've seen the pricing for ANA's before as it shows up on the US website, as long as your buying it a couple months ahead the availability is pretty good.

You'd likely end up paying a bit less on Jetstar if you don't check any bags, but you wouldn't have the lounge access with OW Sapphire or *G (which honestly, lounges aren't much to speak of on domestic flights but better than the terminal). With JL or NH you'd have your full alliance benefits.

Last thing to keep in mind, unless you're planning to transfer directly from your international flight to Sapporo, then HND (Haneda) has far more daily flights and is a lot more convenient to central Tokyo.